Recently, communication services with large communication capacity, including video service for smartphones, have been provided. Accordingly, there is a need for greatly increasing the existing communication capacity and a DWDM (Dense Wavelength Division Multiplexing) type communication has been adopted to increase communication capacity using optical fibers that were installed already. The DWDM is a type that transmits light having several wavelengths through one optical fiber, using a phenomenon that light signals having several wavelengths do not interfere with each other even if they are simultaneously transmitted through one optical fiber, because laser lights having different wavelengths do not interfere with each other.
At present, NG-PON2 (Next Generation-Passive Optical Network version2) is internationally under consideration as a standard and NG-PON2 sets four channel wavelengths for downward optical signals for subscribers from stations at a signal speed of 10 Gbps class. The wavelength spacing of the four channels is set to 100 GHz. Downward wavelengths set with a spacing of 100 GHz under the international standard, NG-PON2 are as follows.
TABLE 1ChannelCentral Frequency (THz)Wavelength (nm)1187.81596.342187.71597.193187.61598.044187.51598.895187.41599.756187.31600.607187.21601.468187.11602.31
An optical receiver is supposed to selectively receive optical signals, which is supposed to be received, in the optical signals simultaneously transmitted through at least four adjacent channels in the set wavelengths, and is supposed not to receive other three optical signals.
For wavelength-selective optical reception of optical receivers, wavelength-tunable optical receivers that can dynamically determine a reception wavelength have been developed. As wavelength-tunable filters used in those wavelength-tunable optical receivers, Fabry-Perot (FP) type etalon filters manufactured by depositing dielectric thin films having high and low refractive indexes on both sides of a semiconductor substrate made of silicon, GaAs, or InP of which the refractive indexes change in accordance with temperature.
FIG. 1 illustrates a wavelength-tunable filter employed in U.S. Pat. No. 6,985,281. The wavelength-tunable filter in the above U.S. patent transmits only specific wavelengths by alternately depositing amorphous silicon and SiO2 on a glass substrate used in this art.
FIG. 2 illustrates a transmissive characteristic curve of an FP type etalon filter. The FP type etalon filter is defined as a wavelength-selective filter having a plurality of transmissive peaks with a predetermined period (FSR: Free spectral range) based on frequency. The FSR (free spectral range) means a wavelength spacing or a frequency spacing between adjacent channels in an FP type etalon filter and is an important factor in the FP type etalon filter.
The FSR is determined by the following Equation 1 in relation with refractive index of the material of a resonator and the length of the resonator in an FR type etalon filter.FSR=c/(2×n×d×cos θ)  [Equation 1]
where c is a light speed, n is a refractive index of the material of a resonator in an etalon filter, d is the incident angle of light on the etalon filter, and θ is the incident angle of light on the etalon filter.
In Equation 1, when the etalon filter is arranged perpendicular to the traveling direction, θ may be considered as “0”, which means cos=1. A rate of change of FSR to the length of a resonator in an etalon filter that is an important variable for manufacturing an etalon filter can be expressed as the following Equation 2.δ(FSR)=c/(2×n×d2)×δd)  [Equation 2]
Accordingly, a ratio relative of change of the FSR according to a little change in length of the resonator can be expressed, by the following Equation 3.δ(FSR)/FSR=δd/d  [Equation 3]
As in Equation 3, the rate of relative change of the FSR is in proportion only to the rate of change in length of the resonator, and the FSR of an etalon filter can be easily adjusted within 1% by precisely adjusting the thickness by 1%.
For example, the length of a resonator in a silicon etalon filter having a spacing of 400 GHz is about 117 um, so when the thickness of the etalon filter is adjusted with precision of 1.17 um that is 1.0% of the length of the resonator, the FSR has precision of 400 GHz +/4 GHz.
FIG. 3 illustrates a process of setting a wavelength and selecting an optical signal at one wavelength by a wavelength-tunable filter in a structure following NG-PON2, in DWDM (dense wavelength division multiplexing) with 4 channels at 100 GHz.
FIG. 3-a illustrates an optical wavelength signal with four channels a, b, c, and d with a spacing of 100 GHz. In FIG. 3-a, the dotted line illustrates a channel used in optical communication and a wavelength having a spacing of 100 GHz and, are not used in actual optical communication. That is, in FIG. 3-a, the dotted line illustrates not actual optical signals, and only a, b, c, and d illustrated by solid lines mean wavelength having an actual communication signal.
As in FIG. 3-a, it is assumed that when four wavelengths simultaneously travel into a wavelength-tunable optical receiving element, a wavelength-tunable filter is an FP type etalon filter having a spacing of at least 400 GHz, as in FIG. 3-b. This embodiment will be described when FSR is 400 GHz. However, there is no problem in use if the FSR of an FP type etalon filter is actually 350 GHz or more. As in FIG. 3-b, it is assumed that the channel a is selected from the wavelengths of four channels, using an FP type etalon filter having an FSR of 400 GHz. In this case, the adjacent transmissive filter band of the FP type etalon filter has a wavelength spacing of at least 100 GHz from the channel d. Therefore, in the four channels a, b, c, and d, it is possible to selectively receive an optical signal only through the channel a, not the channels b, c, and d. Above, an FP type etalon filter having a wavelength spacing of 400 GHz is used. If an FP type etalon filter having an FSR of 380 GHz is used, the adjacent transmissive band of the FP type etalon filter has a wavelength spacing of at least about 80 GHz from the channels a, b, c, and d when the FP type etalon filter receives any one of the channels a, b, c, and d. In general, FP type etalon filters can very easily achieve a −20 dB light isolation line breadth at about 80 GHz, so it is possible to effective receive an optical signal through any one of four 100 GHz channels without a problem, using an FP type etalon filter having an FSR of 380 GHz or 400 GHz. Further, when the FSR is 400 GHz or more, a plurality of transmissive peaks of the FP type etalon filter do not simultaneously select two channels in any cases. Accordingly, FP type etalon filters are available for wavelength-tunable optical receivers as long as the FSR is 380 GHz or more, so thickness of precision of FP type etalon filters is very relaxed in terms of the FSR.
FIG. 3-c illustrates selecting the channel “b” by changing the transmissive wavelength band of a FP type etalon filter having the transmissive wavelength band of FIG. 3-b. In particular, FP type etalon filters based on semiconductor substrates made of silicon, InP, and GaAs increase in refractive index with an increase in temperature, resulting in moving the transmissive wavelength band to a long wavelength. This process can be explained by Bragg's law.m×λ=2×n×d×cos θ  [Equation 4]
where λ is the wavelength of light passing through an FP type etalon filter, m is a positive natural number, n is the refractive index of the material of a resonator in the etalon filter, d is the distance of the resonator in the etalon filter, and θ is an incident angle of light on the etalon filter.
That is, when the temperature of an FP type etalon filter is changed by the Equation 4, the refractive index of the resonator in the etalon filter changes and, the transmissive wavelength band changes accordingly. In general, semiconductor substances such as Si, GaAs, and InP increases in wavelength by about 12 GHz/° C., so a transmissive wavelength transfers to an adjacent channel with a spacing of about 8° C. That is, for example, when the temperature of an FP type etalon filter transmitting the channel “a” is 32° C., the FP type etalon filter transmits the channels “b”, “c”, and “d” at 40° C., 48° C., and 56° C., respectively.
There are two methods, at present, of adjusting the temperature of FP type etalon filters, a method of using a heater and a method of using a thermoelectric element, but both methods cannot adjust temperature within a large range. That is, when a heater is used, it is impossible to operate an etalon filter within a temperature range higher than the temperature of the external environment. For example, when the temperature of an external environment increases up to 80° C., an etalon filter has to operate at 90° C. or more to be able to adjust the temperature. However, considering thermal stability of the material of an etalon filter and package parts, the etalon filter should be operated at as low temperature as possible, and for example, a range of 40° C. from 90 to 130° C. can be set. However, in this case, the minimum temperature range for achieving all of the channels “a”, “b”, “c”, and “d” is 24° C., so the temperature range for selecting the channel “a” should be within 90 to 106° C. to be able to select all the four channels in the range of 90 to 130° C.
If a thermoelectric element is used, similarly, when the temperature is too lower than the temperature of an external environment, power consumption increases, so it is preferable to operate the thermoelectric element within the range of 40 to 80° C. When the transmissive wavelength band of an FP type etalon filter is adjusted by a thermoelectric element, the temperature range of the etalon filter for selecting the channel “a” should be within the range of 40 to 56° C. in order to selectively transmit all channels within the range of 40 to 80° C., using the etalon filter.
In detail, when a thermoelectric element is used, it is possible to selectively transmit all the four channels within the range of 40 to 80° C. of the temperature of the thermoelectric element, only when an FP type etalon filter has the wavelength of the channel “a” and a transmissive wavelength band within +/−0.8 nm at 48° C.
Further, the following Equation 5 can be obtained from Equation 4,δλ/λ=δd/d  [Equation 5]
where, thickness should be adjusted with accuracy of 58 nm to precisely adjust the wavelength with precision of +/−0.8 nm.δd=d×δλ/λ=116 um×0.8 nm/1596 nm=0.058 um=58 nm  [Equation 6]
In Equation 6, an etalon filter having an FSR of 400 GHz and optical communication in 1596 nm band is considered and the allowable wavelength, precision was limited within +/−0.8 nm. The thickness of 58 um cannot be adjusted actually, so the spacing of an etalon filter can be very easily adjusted, whereas the position of the transmissive peak of the etalon filter cannot be adjusted. Accordingly, it is possible, in this case, to select a wavelength-tunable filer by randomly manufacturing an etalon filter and then selecting an etalon filter having a transmissive peak within +/−0.8 nm at a predetermined temperature of the etalon filter. However, in this case, since the etalon filter is randomly manufactured, only a quarter of an FP type etalon filter having a plurality of transmissive peaks has a desired wavelength band, so there is a problem in that a loss increases in the etalon filter. When the operation temperature range of a heater or the operation temperature range of a thermoelectric element to reduce a loss in an etalon filter is increased, it occur problem that power consumption is increased and problem of the thermal stability of the package components.